Spirochetes are helically shaped, motile bacteria. Among the spirochetes, four genera are identified as pathogens, Treponema, Borrelia, Serpulina and Leptospira. The treponemes include Treponema pallidum, the causative agent of the venereal disease syphilis in humans. Other treponemes such as T. denticola and T. vincentii are usually found in the mouth and have been linked to periodontal disease. Several other spirochetes can cause animal as well as human disease. Borrelia recurrentis, which is carried by ticks, causes relapsing fever in which the infected subject suffers repeated bouts of fever and chills. B. burgdorferi, also tick-borne, is responsible for Lyme disease, a debilitating ailment known to cause arthritis, loss of myelin from nerve cells and myocarditis. Serpulina hyodynsenteriae causes outbreaks of diarrhea and weight loss in pigs. Serpulina pilosicoli has recently been identified as a cause of chronic diarrhea in humans. Pathogenic strains of Leptospira frequently cause the zoonotic disease, leptospirosis, and are capable of infecting most mammalian species. At present, there are seven pathogenic species and three nonpathogenic species within the genus Leptospira. Infection occurs either through direct contact with an infected animal or indirect contact with contaminated soil or water. In livestock, the disease causes economic losses due to abortion, stillbirth, infertility, decreased milk production, and death.
Efforts to control leptospirosis have been hampered because virulent leptospires have the capacity for both long-term survival in the environment as well as persistent infection and shedding by wildlife and livestock. Currently available leptospiral vaccines produce short-term immunity and do not provide cross-protection against many of the over 200 different serovars of pathogenic Leptospira (Thiermann, et al., J. Am. Vet. Med. Assoc. 184:722, 1984). These vaccines consist of inactivated whole organisms or outer envelope preparations which produce seroreactivity as determined by microscopic agglutination of intact organisms. The nature of the protective immunogens in these vaccine preparations has not been conclusively elucidated, although several lines of evidence suggest that lipopolysaccharide may confer a degree of serovar-specific protection.
The pathogenesis of leptospirosis is similar to that of other spirochetal diseases, including syphilis (caused by Treponema pallidum) and Lyme borreliosis (caused by Borrelia burgdorferi). Both syphilis and Lyme borreliosis are characterized by widespread dissemination early in the course of disease, including invasion of the central nervous system. Leptospira share this ability with other pathogenic spirochetes such that meningitis is a common manifestation of leptospirosis. Another shared feature of spirochetal infections is the ability to persist chronically in the host, as manifested in cases of tertiary syphilis, chronic Lyme arthritis and leptospiral colonization of the renal tubule in reservoir hosts.
Attempts to identify leptospiral outer membrane proteins (OMPs), have led to limited success due to such problems as: 1) the techniques used to identify surface-exposed proteins probably involved damage to the fragile leptospiral outer membrane resulting in exposure of subsurface structures; 2) putative surface-exposed proteins that were identified included a 35-36 kD doublet corresponding to Leptospira endoflagella (Kelson, et al., J. Med. Microbiol. 26:47, 1988), which are subsurface structures in spirochetes; and 3) use of sodium docecyl sulfate (SDS) which nonselectively solubilizes proteins irrespective of their native cellular location.
Nunes-Edwards, et al. (Infect. Immun. 48:492, 1985) introduced the use of radioimmunoprecipitation and cell fractionation schemes based on the use of SDS in an effort to identify leptospiral OMPs. The leptospires used in their radioimmunoprecipitation procedure were subjected to high speed centrifugation (20,000.times.g) prior to the addition of antibody. Such high centrifugal forces cause mechanical disruption of the leptospiral outer membrane. Niikura, et al. (Zbl. Bakt. Hyg. A. 266:453, 1987) immunoprecipitated SDS-solubilized extracts of virulent and avirulent strains of L. interrogans serovar copenhageni that had been labeled by lactoperoxidase-catalyzed surface radioiodination. Since both of these studies precipitated a 35-36 kD doublet consistent with leptospiral endoflagella, there was a concern as to whether the other proteins identified might also have a subsurface rather than a surface location.
Jost, et al. (J. Med. Microbiol. 27:143) characterized a monoclonal antibody with specificity for a 35 kD proteinase K sensitive antigen which was present in a leptospiral outer envelope preparation. However, to demonstrate binding of the monoclonal antibody by immunoelectron microscopy, the leptospiral outer membrane had to be disrupted. Doherty, et al. (J. Med. Microbiol. 28:143) cloned two leptospiral proteins represented in an SDS-generated outer membrane preparation of L. interrogans, but did not provide corroborating evidence that these proteins are either constituents of the outer membrane or are surface-exposed.
Unsuccessful research on the identification of Leptospira and T. pallidum OMPs has shown the importance of taking into account spirochetal outer membrane fragility and the lack of outer membrane selectivity of ionic detergents such as SDS (Cunningham, et al., J. Bacteriol. 170:5789, 1988; Penn, et al., J. Gen. Microbiol. 131:2349, 1985; Stamm, et al., Infect. Immun. 55:2255, 1987). Outer membrane proteins are of great importance because they play a key role in bacterial pathogenesis. The identification of outer membrane proteins involved in Leptospira pathogenesis is significant to understanding not only leptospiral outer membrane proteins and their involvement in pathogenesis, but also to understanding other spirochetal outer membrane proteins.